Don't know how far and how deep you want to go, but it might be a good idea to:

I. Put Dorset and Somerset de iure under d_Cornwall or, perhaps, a new duchy of Dumnonia. This would, unfortunately, result in a Dumnonia that didn't include Devon(!), but perhaps better than a four-county d_Cornwall? Alternatively, d_Cornwall could be reduced to the 1 county of c_Cornwall, with Devon, Somerset and Dorset forming de iure d_Dumnonia.

The reason is that both Dorset and Somerset were relatively recent conquests as of 769 — they'd been conquered in 700–730 AD, which means already during the lifetime of the 769 Petty King of Cornwall and some of his eldest courtiers (with a 40-year time span, this may include warriors old enough to have fought in those wars but young enough to still be capable of fighting in a reclamation war).

This could result in the following progression for Cornwall: 1. Cornwall => 2. after getting Somerset and Dorest back => restored Petty Kingdom of Dumnonia => 3. after taking one other duchy => capable of creating a custom kingdom on the true king tier.

II. Shake the North up with slightly more Welsh-cultured provinces in recently conquered areas.

III. Shake it all up with adding a claimant or two to d_Gododdin (Lothian) etc. (it's possible, though unlikely, that Gododdin still existed in 769), though a Brythonic claimant to Elmet (d_York) would probably too far a shot after more than a hundred years.

IV. Create some kind of mechanism to enable Anglo-Saxon-cultured provinces to flip to Welsh/Breton (Brythonic) after liberation. This is because the Angles and Saxons are unlikely to have been a real majority, as opposed to just simply a dominant cultural/social/political element. Hence, a change of power, with a Brythonic ruler ending the Saxon privileges, enfranchising the subjugated Welsh peasantry and calling any surviving irredentists from the woods (not too far a shot, since there is some circumstancial evidence of them even in the 10th century) — would probably have meant a 'back flip'. After this, the remaining Saxons could also culture-flip, or migrate, or remain as a Saxon-cultured city or barony (i.e. ruled by a Saxon-cultured vassal).

V. Make a way to integrate with those Welsh who are tribals in de iure k_Pictland, such as moving Lothian and Galloway de iure into your kingdom without waiting 100 years.

VI. Create some kind of decision to (re)create the High Kingship on the basis of the consent of petty kings to concede some power and leadership to the most powerful or otherwise best candidate of all of them? (E.g. opinion, military power, wealth, some traits, prestige, piety, other such possible criteria.)

VII. Create a way to absorb the Saxons rather than physically destroying or outright subgjugating them. Diplovassalization won't normally work because of different culture and small difference in rank, even when you become their de iure liege, so it would take a decision to effect some kind of peace with the Saxons whereby they get to keep some duchies as the vassals of the restored Brythonic kingdom. Doing so would remove de iure k_England and put everything south of the Wall into one de iure kingdom.

I think this is kinda necessary because after 300 years it's not like in Arthur's own time any more. The nation is smaller, and the Saxons are more entrenched, more in a position to claim long-term peaceful possession etc. Hence the need for an elegant solution.

Thanks for your response.

I. I don't want to make one duchy with 1 county and another with 4 counties... I also don't really want to touch the structure of the titles too much

II. I might do that but I'll look into the history first, I'd like to ground any changes reasonably.

III. Claimants sound fine, I'll look up past rulers and maybe make some up if I can't find any.

IV. In my experience cultures change via existing mechanics (the random events) quite nicely, the mean time is about 50 years per province I believe.

V. 100 years isn't too long to wait I'd say. The Anglo-Saxons arrived in 400 AD so they've had 300 years (in the Charlemagne start) to entrench themselves.

VI. I'd say the existing Emprie of Brittania title is ok, although I could look into adding a localisation change (so that Welsh Emperors are called High Kings).

VII. Definitely don't want to make it any easier for the player!

I would like to suggest a celtic reconquest Casus Belli, in which any ruler of Celtic culture can declare a war that targets a single duchy in britain held by an Anglo Saxon or English ruler. A Celtic Unification CB would also be good for forming Britain, though these CB's should cost 1000 or so prestige to use for war.

It would also be nice to have a decision to unify all celtic cultures into one Celtic culture once you conquer Brittany, Scotland, Ireland, Wales and England.

Sorry but I don't want to give free CBs to the player, and I quite like seeing the different Celtic subcultures

Hi again. My comment re: Saxons not being sufficiently entrenched yet concerned only recent conquests as of 769. This means the areas they conquered at dates like 680, 705, 730.

This was also the point of bringing up the usual 100-year period for de iure drift. For example, according to some historians Dorset and Somerset were only conquered by Wessex around 700 AD or even 730 AD , and previously they had belonged to Dumnonia. But in CK2, they start as parts of de iure Kingdom of England in 769, while it would make more sense to put them in de iure Wales (although de facto ruled by Wessex, sure).

In any case, maybe just give the old petty king in Cornwall some personal strong pressed claims?

Re: easier, well, maybe not really easier but some form of recognition that a Celtic reconquest/liberation would be something else than a french conquest from the outside (like, reviving old claims, reaching out to existing Celtic minorities, or to oppressed Celtic majorities under the rule of a minority Saxon elite etc.). Hope this clarifies my suggestions a bit.

This isn't a suggestion but more of a request, would it be possible for me to use your mod in a mod that I'm making? I would of course give you credit and direct people to this mod page.

Sure thing, you can use this mod